Cardiac resynchronization therapy (CRT) is an effective treatment for heart failure patients. One goal of CRT is the production of a mechanically synchronous ventricular contraction.
Studies demonstrate that CRT therapy which provides pacing only to the left ventricle (LV only pacing) produces hemodynamic benefits similar to bi-ventricular pacing. LV only pacing algorithms attempt to achieve electrical fusion by timing the left ventricular pulse (LV pace) based upon sensed electrical signals such as right ventricular intrinsic depolarization. The underlying assumption of such algorithms is that artificial coupling of the LV stimulus and the intrinsic RV depolarization will result in a mechanically synchronous contraction. However, the timing of electrical events may not correlate well with the timing of mechanical events. For example, some forms of CRT attempt to deliver the LV pacing stimulus in such a way that it precedes the RV sensed electrical event by a pre-defined time interval. However, the RV sensed event corresponds to the instant of local electrical activation under the tip of the RV electrode and may not reflect the global ventricular excitation pattern. In addition, there may be variations in conduction velocities or in excitation-contraction coupling among the heart tissues at different locations in the heart. Therefore, timing delivery of a left ventricular pulse based upon measured electrical events such as the RV sensed event may not result in a mechanically synchronous contraction. Therefore, in order to achieve a mechanically fused left ventricular contraction, it is desirable to provide left ventricular stimulation based on the mechanical activity of the heart, sensed using implanted sensors, rather than the electrical activity.